This invention relates to an optical printer in which a photosensitive body is exposed to laser beams applied thereto so as to scan the same therewith one-dimensionally (linearly) is moved in the direction which is at right angles to these laser beams, and more particularly to an optical printer used effectively for applying a plurality of laser beams at once to a photosensitive body so as to scan the same.
In a prior art laser printer, a photosensitive body is generally scanned with a single laser beam by using a rotary polygon mirror. Recently, with a view to satisfying a demand for a higher-speed printing operation, and, owing to the progress of the practical use of a semiconductor laser array, a printer in which a plurality of laser beams are used at once for scanning a photosensitive body therewith has been developed. In a conventional scanning operation using a plurality of laser beams, a one-by-one scanning method is generally employed, in which the scanning is done successively from the upper portion of an image plane by using N laser beams, in such a manner that first to Nth scanning operations are carried out at once initially with N+1st to 2Nth scanning operations carried out at once subsequently. As a matter of fact, in an array using a semiconductor laser, the size of the emitting point is 1-3 .mu..phi.. Therefore, in order to carry out a scanning operation densely, it is necessary to form the array densely and set the intervals among the lasers to around 1-3 .mu.m.phi.. However, if the lasers are put so close to one another, thermal and electric crosstalk occurs, or the optical resonators influence each other, so that each laser stops working as an independent light source. Consequently, it is necessary that the lasers be spaced from one another to a certain extent to minimize the crosstalk among the lasers.
Therefore, it is difficult to set the space between two adjacent emitting points in a laser array used for a conventional laser printer to not more than 50 .mu.m. In order to obtain a neighboring line sequential scanning mode, a method of disposing a laser array 1 incliningly as shown in FIG. 1 is employed (refer to Japanese Patent Laid-Open No. 158251/1979 and its corresponding U.S. Pat. No. 4,253,102).
To practice this method, it is necessary that the laser array as a whole be turned in the substantially horizontal direction to dispose the same at an angle .theta. shown in FIG. 1. However, when this angle .theta. is too large under such circumstances, a scanning line is superposed on the adjacent scanning line or separated therefrom if a very small error occurs in the angle thus set. Therefore, it is necessary to set this angle .theta. to the lowest possible level. During this time, the plane including the emitting points of the lasers and the printing surface form a geometrooptic conjugate point.
Even when a printer formed by using a plurality of laser beam sources in which the lasers are arranged in a discrete manner, and not in the manner as in a laser array, is employed, it is necessary that the positions of the laser beam sources be regulated to correct the variations with the lapse of time of the mutual distances among the laser beam sources. In order to a range the lasers close to one another as mentioned above, it is necessary that the emitting points be arranged densely. Examples of the optical printer in which the lasers are arranged with the emitting points positioned close to one another are disclosed in U.S. Pat. No. 4,644,160 and Japanese Patent Laid-Open No. 166916/1985. FIG. 2 illustrates such an example. The laser beams 21, 22 emitted from two lasers 11, 12 are introduced in substantially the same direction by a prism 10 to an optical system shown in FIG. 2 and consisting of a rotating polygonal mirror 5 and a F.theta. lens 6. Reference numerals 91, 92 denote known actuators, and 41, 42 known controllers for use of controlling these actuators. In order to fixedly space two laser spots 71, 72 on the surface to be scanned, from each other by a predetermined distance, a part of each of the beams from the lasers is introduced into four divisional photodetectors A.sub.1 -A.sub.4 so as to form one spot on the light-receiving portions A.sub.1, A.sub.2 and the other spot on the light-receiving portions A.sub.3, A.sub.4 ; and B.sub.1 -B.sub.2, B.sub.3 -B.sub.4 are then determined as shown in the drawing, by using the output signals (detected signals) B.sub.1 -B.sub.4 obtained from these light-receiving portions. The signals thus obtained are used as spot control signals, and optical reflectors 3, 4 are actuated so as to maintain the spots in predetermined positions constantly stable. During this time, the diameter of the optical spots 71, 72, which are moved in the sub-scanning direction, and that of the spots 771, 772 in the sub-scanning direction on the four divisional photodetectors are maintained so as to have geometrooptical conjugate relation (with a magnification M). Accordingly, the diameter of the photodetectors and the distance between the two spots are equal to the products of the diameter of the spots and the distance between these spots on the surface to be scanned and the magnification M. In the case where a scanning operation is carried out over the neighboring lines by using two laser beams, the four divisional photodetectors are arranged in contact in the subscanning direction as shown in FIG. 3, and laser spots 781, 782 have to be formed accurately on the light-receiving portions (A.sub.1, A.sub.2), (A.sub.3, A.sub.4) as shown in the drawing respectively. There is the possibility that, for example, the spot 781 is formed on the light-receiving portion (A.sub.3, A.sub.4) in a regularly directed state or in a contrariwise directed state due to a certain noise or error. In such a case, crosstalk occurs, and it becomes difficult to control the spots independently of each other. Consequently, it is necessary to solve the problems so that the light-receiving portions (A.sub.1, A.sub.2), (A.sub.3, A.sub.4) can be spaced from each other without causing any troubles.
The problems of the intervals of the scanning lines in the sub-scanning direction in an optical printer using a plurality of laser beams have heretofore been described. The problems of the positional relation in the primary scanning direction between the laser beams will now be additionally described.
A method of detecting the positions of the scanning laser beams in the primary scanning direction on the basis of the time at which the laser beams pass the photodetectors provided at the end of a surface to be scanned has been practically used. The conventional methods of more accurately detecting the positions of the scanning laser beams in the main scanning direction include a system (Japanese Patent Laid-Open No. 61545/1979), in which two divisional detectors 81, 82 separated in the main scanning direction as shown in FIG. 4 are used, a laser beam 52 with which a photosensitive drum 51 has been scanned by a reflector 53 being detected on the basis of outputs obtained from these two detectors 81, 82 by the differential operations thereof, and the position (time) at which the laser beam passes the detectors being determined on the basis of the position of the zero intersection of these outputs. However, in spite of the presence of this system, such two photodetectors are not used but a single photodetector is used in all cases to detect the position of the scanning laser beam on the basis of an output from this photodetector. The reasons why an optical printer using a single photodetector is employed are as follows. When a single laser is used, the level of an output therefrom does not vary in a short period of time, and an operation for detecting the position of the beam is little influenced by this output. Therefore, it is considered that the detection of the position of the laser beam can be done with no particular troubles even by a single detector. However, when a plurality of lasers are used, in which a constant difference between the outputs therefrom occurs, it becomes impossible to determine the positions of the laser beams accurately by a single photodetector. FIG. 5 shows the influence of difference between the outputs from a plurality of lasers. A case where the laser outputs scatter as shown by reference numerals 1001-1003 will now be discussed. If there is a difference between the outputs from a plurality of lasers by this method, in which a threshold value 101 is set in a certain level of the laser outputs to thereby determine the positions of the laser beams, the time signals 1021-1023 obtained from the threshold level 101 are found not to accurately indicate the positions of the laser beams even when the diameters of the optical spots are set equal.
In order to scan a photosensitive body with a plurality of laser beams in a conventional optical printer, it is desirable to carry but a neighboring line sequential scanning operation for the signal processing convenience. In spite of this fact, it is practically very difficult to provide an optical system suitable for the sequential scanning due to various optical and mechanical requirements.